A cold cathode fluorescent light (CCFL) has been increasingly used as a back light source of a liquid crystal display monitor of a notebook PC and of a liquid crystal display (LCD) for use with, for example, a TV set. Such CCFL has substantially the same high efficiency and long life as a usual hot cathode fluorescent light, without using a filament of the hot cathode fluorescent light.
In order to start up and operate the CCFL, a high ac voltage is required. For example, a startup voltage of about 1000 V and an operating voltage of about 600 V are required. These high ac voltages are generated from a dc power supply unit of, for example, a notebook PC and a liquid crystal TV set, using an inverter.
Conventionally, a Royer circuit has been used as an inverter for the CCFL. The Royer circuit comprises a saturable magnetic core transformer and a control transistor, and is adapted to undergo a self-sustaining oscillation owing to the nonlinear permeability of the saturable magnetic core and the nonlinear current gain characteristic of the control transistor. The Royer circuit itself requires no external clock or driver circuit.
However, a Royer circuit is basically a constant-voltage inverter, which cannot maintain a constant output voltage if the input voltage thereto and/or the load current thereof varies. Hence, in order to maintain a constant input voltage to the Royer circuit, a regulator for supplying electric power to the Royer circuit is required. For this reason, the inverter utilizing a Royer circuit cannot be easily miniaturized, and has low power inversion efficiency.
A CCFL inverter having improved power conversion efficiency has been disclosed (see for example Japanese Patent Application H10-50489). This inverter comprises a first semiconductor switch connected in series with the primary winding of a transformer, a serially connected second semiconductor switch and a capacitor which are connected in parallel with the primary winding, and a coupling capacitor and a load connected in series with the secondary winding of the transformer. The first and second semiconductor switches are switched on and off by a control signal received from a control circuit to supply ac power to the load.
A full-bridge (often called H bridge) type CCFL inverter utilizing four semiconductor switches has been proposed (see for example U.S. Pat. No. 6,259,615.) This inverter has a transformer having its primary winding connected to the output end of the full bridge via a resonant capacitor connected in series with the primary winding. The load is connected to the secondary winding of the transformer. Of the four semiconductor switches constituting the full bridge, a first set of two semiconductor switches establishes a current path in a first direction to the primary winding of the transformer and a second set of two semiconductor switches establishes a current path in a second direction to the primary winding. The control circuit provides the full bridge semiconductor switches with control signals each having a fixed pulse width and a controlled relative position of the pulse, thereby regulating the power given to the load. Over-current protection is carried out by detecting the voltage across the secondary winding of the transformer.
Although conventional inverters are designed to drive a load under a constant-current control and protect the load against an excessive voltage, it is often the case that the load, particularly a CCFL in the present example, is subjected to an excessive current and/or excessive voltage during a startup on account of a loop-delay encountered in the constant-current control or an operational delay in protection against an excessive voltage. This excessive current and excessive voltage impose a strong stress on the CCFL, thereby shortening the life of the CCFL. Furthermore, major components of the inverter such as a transformer, a semiconductor switch, and a battery must be constructed to withstand excessive current and excessive voltage.
It is, therefore, an object of the invention to provide an inverter having a semiconductor switch circuit connected to the primary winding of a transformer such that the switches of the semiconductor switch circuit are respectively controlled by pulse-width modulation (PWM) to supply a constant current and a constant voltage to the load, said inverter capable of preventing an excessive inrushing current to the load and an excessive voltage on the load from occurring during a startup, irrespective of a possible loop delay in the constant-current/voltage control loop.
It is another object of the invention to provide a controller IC for use in such inverter.